Wavelength division multiplexed (WDM) optical fiber communication systems are known. Such a system typically comprises wavelength reference means and feedback means that maintain the wavelength of the output radiation of a laser at a predetermined value.
Exemplarily, reference means comprise optical fiber with in-line refractive index grating. See, for instance, U.S. Pat. No. 5,077,816, which discloses a wavelength multiplexed fiber communication system that comprises a narrow band (e.g., 60 GHz tuning range) semiconductor laser and a narrow band (e.g., 44 GHz) refractive index reflection grating, with the feedback loop locking the laser output to the center frequency of the grating. See also C. R. Giles et al., IEEE Photonics Technology Letters, Vol. 2(1), p. 63, which discloses use of an Er-YAG optical filter, and use of a feedback loop to control the current to the Bragg section of a two-section DBR laser.
Some shortcomings are associated with prior art wavelength stabilization schemes. The scheme of the '816 patent involves use of a narrow band laser. However, it would frequently be desirable to be able to use lasers having a broad tuning range such that nominally identical (broad band) lasers could be used as sources in a WDM system, thus avoiding the need to use specially designed lasers for the various channels. The Giles et al. scheme typically does not provide access to all wavelengths within the tuning range of a (broad-band) laser. Such access typically would be desirable. The scheme of the '816 patent typically is not capable of start-up without human intervention ("cold-start") if the wavelength reference means (typically an in-line refractive index grating) does not have a single transmission minimum but exhibits also one or more subsidiary minima, as is frequently the case. It would clearly be desirable to have available a stabilization scheme that is capable of cold-start even under those circumstances. Finally, it would be desirable to have available a stabilization scheme that facilitates some choice in the control frequency, such that (typically unavoidable) small deviations from the target wavelength of the wavelength reference means can be readily compensated for. This application discloses an article that comprises a laser stabilization scheme that mitigates or eliminates at least some of the above recited shortcomings of the prior art schemes.